A transition in which the space-group symmetry of the lower symmetry phase is a subgroup of that of the higher symmetry phase. Example: The transition of the low-temperature polymorph of quartz characterized by space-group symmetry $P{3}_{1}2$ (trigonal), to the high temperature polymorph of quartz with space-group symmetry, $P{6}_{2}22$ (hexagonal). Synonymous with symmetry-breaking transition (note: this term, strictly speaking, is no longer in use).